1. Field of the Invention
The present invention relates to sensors for the detection of metallic debris in dielectric fluids using high frequency electromagnetic fields and more particularly to an inductive debris monitor (IDM) having a sensor system with an extended operating temperature range and improved reliability.
2. Description of the Prior Art
Sensors are known for detecting metallic debris in fluid flow passageways such as in engine and transmission lubricating systems where the presence of metal particles in the dielectric fluid lines can be an indication of the impending failure of the system. Sensing systems of this type are disclosed in U.S. Pat. Nos. 4,926,120 and 5,041,856, issued to W. A. Veronesi et al and assigned to United Technologies Corp., and in U.S. Pat. No. 5,357,197 of Lev Sorkin and co-pending U.S. application Ser. No. 08/143,846 of Gerald Pulice, both assigned to the same assignee as the present application. It is common in such systems, as in the system disclosed, for example, in U.S. Pat. No. 4,926,120, to use a radio frequency (RF) bridge with the sensor to detect changes in impedance in the fluid line being monitored. Under normal operating conditions the RF bridge is automatically balanced with a voltage controlled resistor (VCR) that is implemented by means of a junction field effect silicon transistor. In particular, as shown in FIG. 3 of the exemplary patent, the RF bridge consists of a TRANSFORMER 19, 20, 21, a TANK CIRCUIT 12, 16, 18 and a VOLTAGE CONTROLLED RESISTOR 34. Among the limitations of this arrangement are the maximum operating junction temperature and the limited range of resistance as a function of the voltage control. For the RF bridge to operate over the extended temperature environment required for practical metallic debris detection, the silicon VCR poses a performance problem. Assuming a silicon-carbide based technology were substituted for the junction field effect transistor, such an arrangement would have a maximum permissible operating junction temperature of about 350.degree. C., in contrast to the maximum temperature of about 175.degree. C. for the silicon based device. However, the limited range of resistance control would still pose a technical limit.
It is therefore a problem in the art to achieve a desirable range of resistance control in IDM devices over an extended operating temperature range.
It is accordingly an object of the present invention to provide an improved metallic particle sensor and bridge arrangement that overcomes the operating temperature range and reliability limitations of the prior art IDM sensor systems.
It is another object of the invention to provide an improved means for balancing the RF bridge in IDM devices that enables their operation over a very wide range of temperatures without reliability problems or degradation.
It is also an object of the invention to provide a bridge arrangement that will provide improved operation with IDM devices wherein the resonance impedance decreases with increasing temperature.
It is a further object of the invention to provide a sensor and bridge arrangement with minimum interactions between the bridge circuit and the surrounding environment.